Video encoding device, video decoding device, video encoding method, video decoding method, and program

ABSTRACT

A video decoding device for decoding video using inter prediction comprises decoding control unit setting partition type of CU to be decoded to a type other than N×N which indicates a size of PU obtained by dividing a CU to be decoded is a minimum size, when a prediction mode of the CU to be decoded is an inter prediction and a size of the CU to be decoded is equal to a minimum CU size.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. application Ser.No. 13/979,592 filed Jul. 12, 2013, claiming priority from NationalStage Application No. PCT/JP2012/000046 filed on Jan. 1, 2005, whichclaims priority under 35 USC §119 from Japanese Patent Application Nos.2011-004964 filed Jan. 13, 2011, the contents of all of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a video encoding device, a videodecoding device, a video encoding method, a video decoding method, and aprogram that use hierarchical coding units.

BACKGROUND ART

Non Patent Literature (NPL) 1 discloses typical video encoding systemand video decoding system.

A video encoding device described in NPL 1 has a structure as shown inFIG. 15. The video encoding device shown in FIG. 15 is called a typicalvideo encoding device below.

Referring to FIG. 15, the structure and operation of the typical videoencoding device that receives each frame of digitized video as input andoutputs a bitstream are described below.

The video encoding device shown in FIG. 15 includes atransformer/quantizer 101, an entropy encoder 102, an inversetransformer/inverse quantizer 103, a buffer 104, a predictor 105, amultiplexer 106, and an encoding controller 108.

The video encoding device shown in FIG. 15 divides each frame intoblocks of 16×16 pixel size called macro blocks (MBs), and encodes eachMB sequentially from top left of the frame.

FIG. 16 is an explanatory diagram showing an example of block divisionin the case where the frame has a spatial resolution of QCIF (QuarterCommon Intermediate Format). The following describes the operation ofeach unit while focusing only on pixel values of luminance forsimplicity's sake.

A prediction signal supplied from the predictor 105 is subtracted fromthe block-divided input video, and the result is input to thetransformer/quantizer 101 as a prediction error image. There are twotypes of prediction signals, namely, an intra prediction signal and aninter prediction signal. The inter prediction signal is also called aninter-frame prediction signal.

Each of the prediction signals is described below. The intra predictionsignal is a prediction signal generated based on an image of areconstructed picture that has the same display time as a currentpicture stored in the buffer 104.

Referring to 8.3.1 Intra_(—)4×4 prediction process for luma samples,8.3.2 Intra_(—)8×8 prediction process for luma samples, and 8.3.3Intra_(—)16×16 prediction process for luma samples in NPL 1, intraprediction of three block sizes, i.e. Intra_(—)4×4, Intra_(—)8×8, andIntra_(—)16×16, are available.

Intra_(—)4×4 and Intra_(—)8×8 are respectively intra prediction of 4×4block size and 8×8 block size, as can be understood from (a) and (c) inFIG. 17. Each circle (∘) in the drawing represents a reference pixelused for intra prediction, i.e., a pixel of the reconstructed picturehaving the same display time as the current picture.

In intra prediction of Intra_(—)4×4, reconstructed peripheral pixels aredirectly set as reference pixels, and used for padding (extrapolation)in nine directions shown in (b) of FIG. 17 to form the predictionsignal. In intra prediction of Intra_(—)8×8, pixels obtained bysmoothing peripheral pixels of the image of the reconstructed picture bylow-pass filters (½, ¼, ½) shown under the right arrow in (c) of FIG. 17are set as reference pixels, and used for extrapolation in the ninedirections shown in (b) of FIG. 17 to form the prediction signal.

Similarly, Intra_(—)16×16 is intra prediction of 16×16 block size, ascan be understood from (a) in FIG. 18. Like in FIG. 17, each circle (o)in the drawing represents a reference pixel used for intra prediction,i.e., a pixel of the reconstructed picture having the same display timeas the current picture. In intra prediction of Intra_(—)16×16,peripheral pixels of the image of the reconstructed picture are directlyset as reference pixels, and used for extrapolation in four directionsshown in (b) of FIG. 18 to form the prediction signal.

Hereafter, an MB and a block encoded using the intra prediction signalare called an intra MB and an intra block, respectively, i.e., a blocksize of intra prediction is called an intra prediction block size, and adirection of extrapolation is called an intra prediction direction. Theintra prediction block size and the intra prediction direction areprediction parameters related to intra prediction.

The inter prediction signal is a prediction signal generated from animage of a reconstructed picture different in display time from the onethe current picture has and is stored in the buffer 104. Hereafter, anMB and a block encoded using the inter prediction signal are called aninter MB and an inter block, respectively. A block size of interprediction (inter prediction block size) can be selected from, forexample, 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, and 4×4.

FIG. 19 is an explanatory diagram showing an example of inter predictionusing 16×16 block size. A motion vector MV=(mv_(x), mv_(y)) shown inFIG. 19 is a prediction parameter of inter prediction, which indicatesthe amount of parallel translation of an inter prediction block (interprediction signal) of a reference picture relative to a block to beencoded. In AVC, prediction parameters of inter prediction include notonly a direction of inter prediction representing a direction of thereference picture of an inter prediction signal relative to a picture tobe encoded of the block to be encoded, but also a reference pictureindex for identifying the reference picture used for inter prediction ofthe block to be encoded. This is because, in AVC, multiple referencepictures stored in the buffer 104 can be used for inter prediction.

In AVC inter prediction, a motion vector can be calculated at ¼-pixelaccuracy. FIG. 20 is an explanatory diagram showing interpolationprocessing for luminance signals in motion-compensated prediction. InFIG. 20, A represents a pixel signal at an integer pixel position, b, c,d represent pixel signals at decimal pixel positions with ½-pixelaccuracy, and e₁, e₂, e₃ represent pixel signals at decimal pixelpositions with ¼-pixel accuracy. The pixel signal b is generated byapplying a six-tap filter to pixels at horizontal integer pixelpositions. Likewise, the pixel signal c is generated by applying thesix-tap filter to pixels at vertical integer pixel positions. The pixelsignal d is generated by applying the six-tap filter to pixels athorizontal or vertical decimal pixel positions with ½-pixel accuracy.The coefficients of the six-tap filter are represented as [1, −5, 20,20, −5, 1]/32. The pixel signals e₁, e₂, and e₃ are generated byapplying a two-tap filter [1, 1]/2 to pixels at neighboring integerpixel positions or decimal pixel positions, respectively.

A picture encoded by including only intra MBs is called an I picture. Apicture encoded by including not only intra MBs but also inter MBs iscalled a P picture. A picture encoded by including inter MBs that usenot only one reference picture but two reference pictures simultaneouslyfor inter prediction is called a B picture. In the B picture, interprediction in which the direction of the reference picture of the interprediction signal relative to the picture to be encoded of the block tobe encoded is past is called forward prediction, inter prediction inwhich the direction of the reference picture of the inter predictionsignal relative to the picture to be encoded of the block to be encodedis future is called backward prediction, and inter predictionsimultaneously using two reference pictures involving both the past andthe future is called bidirectional prediction. The direction of interprediction (inter prediction direction) is a prediction parameter ofinter prediction.

In accordance with an instruction from the encoding controller 108, thepredictor 105 compares an input video signal with a prediction signal todetermine a prediction parameter that minimizes the energy of aprediction error image block. The encoding controller 108 supplies thedetermined prediction parameter to the entropy encoder 102.

The transformer/quantizer 101 frequency-transforms the image (predictionerror image) from which the prediction signal has been subtracted to geta frequency transform coefficient.

The transformer/quantizer 101 further quantizes the frequency transformcoefficient with a predetermined quantization step width Qs. Hereafter,the quantized frequency transform coefficient is called a transformquantization value.

The entropy encoder 102 entropy-encodes the prediction parameters andthe transform quantization value. The prediction parameters areinformation associated with MB and block prediction, such as predictionmode (intra prediction, inter prediction), intra prediction block size,intra prediction direction, inter prediction block size, and motionvector mentioned above.

The inverse transformer/inverse quantizer 103 inverse-quantizes thetransform quantization value with the predetermined quantization stepwidth Qs. The inverse transformer/inverse quantizer 103 further performsinverse frequency transform of the frequency transform coefficientobtained by the inverse quantization. The prediction signal is added tothe reconstructed prediction error image obtained by the inversefrequency transform, and the result is supplied to the buffer 104.

The buffer 104 stores the reconstructed image supplied. Thereconstructed image for one frame is called a reconstructed picture.

The multiplexer 106 multiplexes and outputs the output data of theentropy encoder 102 and coding parameters.

Based on the operation described above, the multiplexer 106 in the videoencoding device generates a bitstream.

A video decoding device described in NPL 1 has a structure as shown inFIG. 21. Hereafter, the video decoding device shown in FIG. 21 is calleda typical video decoding device.

Referring to FIG. 21, the structure and operation of the typical videodecoding device that receives the bitstream as input and outputs adecoded video frame is described.

The video decoding device shown in FIG. 21 includes a de-multiplexer201, an entropy decoder 202, an inverse transformer/inverse quantizer203, a predictor 204, and a buffer 205.

The de-multiplexer 201 de-multiplexes the input bitstream and extractsan entropy-encoded video bitstream.

The entropy decoder 202 entropy-decodes the video bitstream. The entropydecoder 202 entropy-decodes the MB and block prediction parameters andthe transform quantization value, and supplies the results to theinverse transformer/inverse quantizer 203 and the predictor 204.

The inverse transformer/inverse quantizer 203 inverse-quantizes thetransform quantization value with the quantization step width. Theinverse transformer/inverse quantizer 203 further performs inversefrequency transform of the frequency transform coefficient obtained bythe inverse quantization.

After the inverse frequency transform, the predictor 204 generates aprediction signal using an image of a reconstructed picture stored inthe buffer 205 based on the entropy-decoded MB and block predictionparameters.

After the generation of the prediction signal, the prediction signalsupplied from the predictor 204 is added to a reconstructed predictionerror image obtained by the inverse frequency transform performed by theinverse transformer/inverse quantizer 203, and the result is supplied tothe buffer 205 as a reconstructed image.

Then, the reconstructed picture stored in the buffer 205 is output as adecoded image (decoded video).

Based on the operation described above, the typical video decodingdevice generates the decoded image.

CITATION LIST Non Patent Literatures

-   NPL 1: ISO/IEC 14496-10 Advanced Video Coding-   NPL 2: “Test Model under Consideration,” Document: JCTVC-B205, Joint    Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 and    ISO/IEC JTC1/SC29/WG11 2nd Meeting: Geneva, CH, 21-28 Jul., 2010

SUMMARY OF INVENTION Technical Problem

NPL 2 discloses Test Model under Consideration (TMuC). Unlike thatdisclosed in NPL 1, the TMuC uses hierarchical coding units (Coding TreeBlocks (CTBs)) shown in FIG. 22. In this specification, CTB blocks arecalled Coding Units (CUs).

Here, the largest CU is called the Largest Coding Unit (LCU), and thesmallest CU is called the Smallest Coding Unit (SCU). In the TMuCscheme, the concept of Prediction Unit (PU) is introduced as a unit ofprediction for each CU (see FIG. 23). The PU is a basic unit ofprediction, and eight PU partition types {2N×2N, 2N×N, N×2N, N×N, 2N×nU,2N×nD, nL×2N, nR×2N} shown in FIG. 23 are defined. The PU used for interprediction is called an inter PU and the PU used for intra prediction iscalled intra PU. The PU partition for which inter prediction is used iscalled inter-PU partition, and the PU partition for which intraprediction is used is called intra-PU partition. Among the shapes shownin FIG. 23, only the squares of 2N×2N and N×N are supported as theintra-PU partitions. Hereafter, the lengths of one side of a CU and a PUare called CU size and PU size, respectively.

The TMuC scheme can use a filter with up to twelve taps to seek for apredicted image with a decimal accuracy. The relationship between pixelposition and filter coefficient is as follows.

TABLE 1 Pixel Position Filter Coefficient ¼ {−1, 5, −12, 20, −40, 229,76, −32, 16, −8, 4, −1} ½ {−1, 8, −16, 24, −48, 161, 161, −48, 24, −16,8, −1} ¾ {−1, 4, −8, 16, −32, 76, 229, −40, 20, −12, 5, −1}

The pixel position is described with reference to FIG. 24. In FIG. 24,it is assumed that A and E are pixels at integer pixel positions. Inthis case, b is a pixel at ¼-pixel position, c is a pixel at ½-pixelposition, and d is a pixel at ¾ pixel position. The same applies tothose in the vertical direction.

The pixel b or pixel c shown in FIG. 20 is generated by applying afilter for horizontal or vertical ½-pixel position once. The pixel e₁ isgenerated by applying a filter for ¼-pixel position once.

Referring to FIG. 25, a description is made of an example of generationof decimal pixels, such as pixel e₂ and pixel e₃, the pixel positions ofwhich are decimal-accuracy positions in both the horizontal and verticaldirections and at least either of which is ¼-pixel position. In FIG. 25,it is assumed that pixel A is a pixel at an integer pixel position andpixel c is a pixel at a decimal pixel position to be obtained. In thiscase, pixel b is first generated by applying a filter for vertical¼-pixel position. Then, pixel c is generated by applying a filter forhorizontal ¾ pixel position relative to the decimal pixel b. In 8.3Interpolation Methods of NPL 2, the generation of decimal pixels isdescribed in more detail.

In the TMuC scheme, a syntax indicative of a PU partition type in eachPU header of CUs on all the levels (according to 4.1.10 Prediction unitsyntax in NPL 2, intra_split_flag in the case of intra prediction andinter_partitioning_idc in the case of inter prediction) is embedded inan output bitstream. Hereafter, intra_split_flag syntax is called anintra-PU partition type syntax, and inter_partitioning_idc syntax iscalled an inter-PU partition type syntax.

When many small-size CUs exist within each LCU, the ratio of the numberof bits of the inter-PU partition type syntax included in the bitstreamincreases, causing a problem that the quality of compressed video isreduced.

Further, in the TMuC scheme, memory accesses to reference picturesincrease as the size of the inter-PU partition becomes smaller, causinga problem of straining the memory bandwidth. Particularly, since thetwelve-tap filter is used to generate a decimal pixel in the TMuCscheme, the memory bandwidth is more strained.

FIG. 26 is an explanatory diagram for describing memory access areaswhen the twelve-tap filter is used. FIG. 26(A) shows a memory accessarea of one inter-PU partition when the PU partition type of N×N isselected, and FIG. 26(B) shows a memory access area when the inter-PUpartition type of 2N×2N is selected.

When N×N is selected, since memory access of a size surrounded by thebroken line in FIG. 26(A) is performed four times in total for each ofinter-PU partitions 0, 1, 2, 3, the amount of memory access has a valueobtained by multiplying 4(N+11)²=4N²+88N+484 by the bit count of areference picture. Since the amount of memory access of the 2N×2Ninter-PU partition has a value obtained by multiplying(2N+11)²=4N²+44N+121 by the bit count of the reference picture, theamount of memory access of the N×N inter-PU partition becomes greaterthan the amount of memory access of 2N×2N.

For example, the amount of memory access of inter PUs in an 8×8 CU whenN=4, the prediction is one-way prediction, and the bit accuracy of eachpixel value is 8 bits is considered. The amount of memory access in the2N×2N inter-PU partition is 19×19×1×8 bits=2888 bits, while the amountof memory access in the N×N inter-PU partition is 15×15×4×8 bits=7200bits, whose amount of memory access is about 2.5 times.

In units of LCU, if the block size of LCU is 128×128, the amount ofmemory access when the LCU is predicted by one inter-PU partition willbe 139×139×1×8 bits=154568 bits, while the amount of memory access whenthe LCU is all predicted by 4×4 inter-PU partitions (i.e., when the LCUis predicted by 1024 inter-PU partitions) will be 15×15×1024×8bits=1843200 bits, whose amount of memory access is about twelve times.

It is an object of the present invention to reduce the memory bandwidthper predetermined area.

Solution to Problem

A video decoding device according to the present invention is a videodecoding device for decoding video using inter prediction, whichincludes decoding control means setting partition type of CU to bedecoded to a type other than N×N which indicates a size of PU obtainedby dividing a CU to be decoded is a minimum size, when a prediction modeof the CU to be decoded is an inter prediction and a size of the CU tobe decoded is equal to a minimum CU size.

A video decoding method according to the present invention is a videodecoding method for decoding video using inter prediction, whichincludes setting partition type of CU to be decoded to a type other thanN×N which indicates a size of PU obtained by dividing a CU to be decodedis a minimum size, when a prediction mode of the CU to be decoded is aninter prediction and a size of the CU to be decoded is equal to aminimum CU size.

A video decoding program according to the present invention causes acomputer for decoding video using inter prediction to execute a processof setting partition type of CU to be decoded to a type other than N×Nwhich indicates a size of PU obtained by dividing a CU to be decoded isa minimum size, when a prediction mode of the CU to be decoded is aninter prediction and a size of the CU to be decoded is equal to aminimum CU size.

Advantageous Effects of Invention

According to the present invention, the use of small inter-PU partitionscan be restricted to reduce the memory bandwidth.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a video encoding device in ExemplaryEmbodiment 1.

FIG. 2 is a flowchart showing a process of determining PU partition typecandidates.

FIG. 3 is an explanatory diagram of a list indicative of information onthe minimum inter-PU size in a sequence parameter set.

FIG. 4 is a flowchart showing a PU header writing operation.

FIG. 5 is an explanatory diagram of a list indicative of information oninter_partitioning_idc syntax in a PU syntax.

FIG. 6 is a block diagram of a video decoding device in ExemplaryEmbodiment 2.

FIG. 7 is a flowchart showing a PU header parsing operation.

FIG. 8 is an explanatory diagram of a list indicative of information onthe minimum inter-PU size in a picture parameter set.

FIG. 9 is an explanatory diagram of a list indicative of information onthe minimum inter-PU size in a slice header.

FIG. 10 is a block diagram of a video decoding device in ExemplaryEmbodiment 4.

FIG. 11 is a flowchart showing an error detection operation.

FIG. 12 is a block diagram showing a configuration example of aninformation processing system capable of implementing the functions of avideo encoding device and a video decoding device according to thepresent invention.

FIG. 13 is a block diagram showing a main part of a video encodingdevice according to the present invention.

FIG. 14 is a block diagram showing a main part of a video decodingdevice according to the present invention.

FIG. 15 is a block diagram of a typical video encoding device.

FIG. 16 is an explanatory diagram showing an example of block division.

FIG. 17 is an explanatory diagram for describing intra prediction ofIntra_(—)4×4 and Intra_(—)8×8.

FIG. 18 is an explanatory diagram for describing intra prediction ofIntra_(—)16×16.

FIG. 19 is an explanatory diagram showing an example of interprediction.

FIG. 20 is an explanatory diagram showing interpolation processing forluminance signals in motion-compensated prediction.

FIG. 21 is a block diagram of a typical video decoding device.

FIG. 22 is an explanatory diagram for describing a CTB.

FIG. 23 is an explanatory diagram for describing a PU.

FIG. 24 is an explanatory diagram for describing decimal pixelpositions.

FIG. 25 is an explanatory diagram for describing a decimal pixelgeneration method using a twelve-tap filter in a TMuC scheme.

FIG. 26 is an explanatory diagram for describing a memory access rangewhen a decimal pixel is generated using the twelve-tap filter.

DESCRIPTION OF EMBODIMENTS

In order to solve the technical problems of the above-mentioned typicaltechniques, the present invention restricts inter-PU partitions based onthe CU depth (i.e. CU size) in video encoding using hierarchical codingunits to solve the problems. In an example of the present invention, theCU size capable of using inter-PU partitions other than 2N×2N isrestricted to solve the problems. In another example of the presentinvention, transmission of an inter-PU partition type syntax in a PUheader is restricted to solve the problems. In the above example of thepresent invention, the ratio of the number of bits of the inter-PUpartition type syntax included in a bitstream can be kept low tosuppress the memory bandwidth while improving the quality of compressedvideo.

Exemplary Embodiment 1

Exemplary Embodiment 1 shows a video encoding device including: encodingcontrol means for controlling an inter-PU partition type based on apredetermined minimum inter-PU size set from the outside; and means forembedding, in a bitstream, information on the minimum inter-PU size tosignal the information on the minimum inter-PU size to a video decodingdevice.

In this exemplary embodiment, it is assumed that available CU sizes are128, 64, 32, 16, and 8 (i.e., the LCU size is 128 and the SCU size is8), and the minimum inter-PU size (minInterPredUnitSize) is 8.

It is further assumed in the exemplary embodiment that the informationon the minimum inter-PU size (min_inter_pred_unit_hierarchy_depth) isbase-2 log (logarithm) of a value obtained by dividing the minimuminter-PU size (8) by the SCU size (8). Thus, in the exemplaryembodiment, the value of min_inter_pred_unit_hierarchy_depth multiplexedinto the bitstream is 0 (=log₂(8/8)).

As shown in FIG. 1, the video encoding device in the exemplaryembodiment includes a transformer/quantizer 101, an entropy encoder 102,an inverse transformer/inverse quantizer 103, a buffer 104, a predictor105, a multiplexer 106, and an encoding controller 107, like the typicalvideo encoding device shown in FIG. 15.

The video encoding device in the exemplary embodiment shown in FIG. 1differs from the video encoding device shown in FIG. 15 in thatminInterPredUnitSize is supplied to the encoding controller 107 totransmit an inter-PU partition type syntax in a CU size greater thanminInterPredUnitSize, and minInterPredUnitSize is also supplied to themultiplexer 106 to signal minInterPredUnitSize to the video decodingdevice.

The encoding controller 107 has the predictor 105 calculate a cost(Rate-Distortion cost: R-D cost) calculated from a coding distortion(the energy of an error image between an input signal and areconstructed picture) and a generated bit count. The encodingcontroller 107 determines a CU splitting pattern in which the R-D costis minimized (the splitting pattern determined by split_coding_unit_flagas shown in FIG. 22), and prediction parameters of each CU. The encodingcontroller 107 supplies determined split_coding_unit_flag and theprediction parameters of each CU to the predictor 105 and the entropyencoder 102. The prediction parameters are information associated withprediction of a CU to be encoded, such as prediction mode (pred_mode),intra-PU partition type (intra_split_flag), intra prediction direction,inter-PU partition type (inter_partitioning_idc), and motion vector.

As an example, the encoding controller 107 in the exemplary embodimentselects the optimum PU partition type as a prediction parameter for a CUwhose size is greater than minInterPredUnitSize from a total of tentypes of intra prediction {2N×2N, N×N}, and inter prediction {2N×2N,2N×N, N×2N, N×N, 2N×nU, 2N×nD, nL×2N, nR×2N}. For a CU whose size isequal to minInterPredUnitSize, the encoding controller 107 selects theoptimum PU partition type as a prediction parameter from a total ofthree types of intra prediction {2N×2N, N×N} and inter prediction{2N×2N}. For a CU whose size is less than minInterPredUnitSize, theencoding controller 107 selects the optimum PU partition type as aprediction parameter from two types of intra prediction {2N×2N, N×N}.

FIG. 2 is a flowchart showing the operation of the encoding controller107 in the exemplary embodiment to determine PU partition typecandidates.

As shown in FIG. 2, when determining in step S101 that the CU size of aCU to be encoded is greater than minInterPredUnitSize, the encodingcontroller 107 sets PU partition type candidates in step S102 to a totalof ten types of intra prediction {2N×2N, N×N} and inter prediction{2N×2N, 2N×N, N×2N, N×N, 2N×nU, 2N×nD, nL×2N, nR×2N}, and determines instep S106 a prediction parameter based on the R-D cost.

When determining in step S101 that the CU size of the CU to be encodedis less than or equal to minInterPredUnitSize, the encoding controller107 proceeds to step S103.

When determining in step S103 that the CU size of the CU to be encodedis equal to minInterPredUnitSize, the encoding controller 107 sets PUpartition type candidates in step S104 to a total of three types ofintra prediction {2N×2N, N×N} and inter prediction {2N×2N}, anddetermines in step S106 a prediction parameter based on the R-D cost.

When determining in step S103 that the CU size of the CU to be encodedis less than minInterPredUnitSize, the encoding controller 107 sets PUpartition type candidates in step S105 to two types of intra prediction{2N×2N, N×N}, and determines in step S106 the optimum PU partition typeas a prediction parameter based on the R-D cost.

The predictor 105 selects a prediction signal corresponding to theprediction parameters of each CU determined by the encoding controller107.

The prediction signal supplied from the predictor 105 is subtracted frominput video of each CU in a shape determined by the encoding controller107 to generate a prediction error image, and the prediction error imageis input to the transformer/quantizer 101.

The transformer/quantizer 101 frequency-transforms the prediction errorimage to obtain a frequency transform coefficient.

The transformer/quantizer 101 further quantizes the frequency transformcoefficient with a predetermined quantization step width Qs to obtain atransform quantization value.

The entropy encoder 102 entropy-encodes split_coding_unit_flag (see FIG.22) supplied from the encoding controller 107, the predictionparameters, and the transform quantization value supplied from thetransformer/quantizer 101.

The inverse transformer/inverse quantizer 103 inverse-quantizes thetransform quantization value with the predetermined quantization stepwidth Qs. The inverse transformer/inverse quantizer 103 further performsinverse frequency transform of the frequency transform coefficientobtained by the inverse quantization. The prediction signal is added tothe reconstructed prediction error image obtained by the inversefrequency transform, and the result is supplied to the buffer 104.

The multiplexer 106 multiplexes and outputs the information on theminimum inter-PU size (min_inter_pred_unit_hierarchy_depth) and outputdata of the entropy encoder 102. According to 4.1.2 Sequence parameterset RBSP syntax in NPL 2, the multiplexer 106 multiplexes log2_min_coding_unit_size_minus3 syntax andmin_inter_pred_unit_hierarchy_depth syntax aftermax_coding_unit_hierarchy_depth syntax in a sequence parameter set aslisted in FIG. 3 (base-2 log (logarithm) of a value obtained by dividingminInterPredUnitSize by the SCU size, i.e. 0 in the exemplaryembodiment). The log 2_min_coding_unit_size_minus3 syntax and themax_coding_unit_hierarchy_depth syntax are information for determiningan SCU size (minCodingUnitSize) and an LCU size (maxCodingUnitSize),respectively. MinCodingUnitSize and maxCodingUnitSize are respectivelycalculated as follows.

       minCodingUnitSize = 1<<       (log2_min_coding_unit_size_minus3 + 3)        maxCodingUnitSize =  1<< (log2_min_coding_unit_size_minus3  +  3  +max_coding_unit_hierarchy_depth)

The min_inter_pred_unit_hierarchy_depth syntax and minCodingUnitSizehave the following relation.

       min_inter_pred_unit_hierarchy_depth   =log₂(minInterPredUnitSize/minCodingUnitSize)

Based on the operation described above, the video encoding deviceaccording to this invention generates a bitstream.

Based on a predetermined minimum inter-PU size and a CU size of a CU tobe encoded, the video encoding device in the exemplary embodimentcontrols the inter-PU partition of the CU to be encoded so that no interPU the size of which is less than the minimum inter-PU size will notcome into existence.

The memory bandwidth is reduced by preventing any inter PU the size ofwhich is less than the minimum inter-PU size from coming into existence.Further, since the number of inter-PU partition type syntaxes to besignaled is reduced by preventing any inter PU the size of which is lessthan the minimum inter-PU size from coming into existence, thepercentage of the amount of code of a PU header in the bitstream isreduced, and hence the quality of video is improved.

The encoding control means in the video encoding device of the exemplaryembodiment controls inter-PU partitions based on the predeterminedminimum inter-PU size set from the outside. As an example, the encodingcontrol means controls inter-PU partition types other than 2N×2N to beused only in CUs of CU sizes greater than a predetermined size.Therefore, since the probability of occurrence of the 2N×2N inter-PUpartition increases to reduce entropy, the efficiency ofentropy-encoding is improved. Thus, the quality of compressed video canbe maintained while reducing the memory bandwidth.

Likewise, for video decoding, the video encoding device in the exemplaryembodiment includes means for embedding, in a bitstream, information onthe predetermined minimum inter-PU size set from the outside so that theinter-PU partition type syntax can be parsed from the bitstream. Thus,since the predetermined size is signaled to the video decoding device,the interoperability of the video encoding device and the video decodingdevice can be enhanced.

Exemplary Embodiment 2

A video encoding device in Exemplary Embodiment 2 includes: encodingcontrol means for controlling an inter-PU partition type based on apredetermined minimum inter-PU size set from the outside and forcontrolling entropy-encoding of an inter-PU partition type syntax basedon the above predetermined minimum inter-PU size; and means forembedding, in a bitstream, information on the minimum inter-PU size tosignal the information on the above minimum inter-PU size to a videodecoding device.

In this exemplary embodiment, it is assumed that the CU size of a CU totransmit the inter-PU partition type syntax is greater than the aboveminimum inter-PU size (minInterPredUnitSize). It is also assumed in theexemplary embodiment that available CU sizes are 128, 64, 32, 16, and 8(i.e., the LCU size is 128 and the SCU size is 8), andminInterPredUnitSize is 8. Thus, in the exemplary embodiment, the CUsizes for embedding the inter-PU partition type syntax in the bitstreamare 128, 64, 32, and 16.

It is further assumed in the exemplary embodiment that information onthe minimum inter-PU size (min_inter_pred_unit_hierarchy_depth) isbase-2 log (logarithm) of a value obtained by dividing the minimuminter-PU size (8) by the SCU size (8). Thus, in the exemplaryembodiment, the value of min_inter_pred_unit_hierarchy_depth multiplexedinto the bitstream is 0(=log₂(8/8)).

The structure of the video encoding device in the exemplary embodimentis the same as the structure of the video encoding device in ExemplaryEmbodiment 1 shown in FIG. 1.

As shown in FIG. 1, the video encoding device in this exemplaryembodiment differs from the video encoding device shown in FIG. 15 inthat minInterPredUnitSize is supplied to the encoding controller 107 totransmit an inter-PU partition type syntax in a CU size greater thanminInterPredUnitSize, and minInterPredUnitSize is also supplied to themultiplexer 106 to signal minInterPredUnitSize to the video decodingdevice.

The encoding controller 107 has the predictor 105 calculate the R-D costcalculated from a coding distortion (the energy of an error imagebetween an input signal and a reconstructed picture) and a generated bitcount. The encoding controller 107 determines a CU splitting pattern inwhich the R-D cost is minimized (the splitting pattern determined bysplit_coding_unit_flag as shown in FIG. 22), and prediction parametersof each CU. The encoding controller 107 supplies the determinedsplit_coding_unit_flag and prediction parameters of each CU to thepredictor 105 and the entropy encoder 102. The prediction parameters areinformation associated with prediction of a CU to be encoded, such asprediction mode (pred_mode), intra-PU partition type (intra_split_flag),intra prediction direction, inter-PU partition type(inter_partitioning_idc), and motion vector.

Like in Exemplary Embodiment 1, the encoding controller 107 in theexemplary embodiment selects the optimum PU partition type as aprediction parameter for a CU whose size is greater thanminInterPredUnitSize from a total of ten types of intra prediction{2N×2N, N×N} and inter prediction {2N×2N, 2N×N, N×2N, N×N, 2N×nU, 2N×nD,nL×2N, nR×2N}. For a CU whose size is equal to minInterPredUnitSize, theencoding controller 107 selects the optimum PU partition type as aprediction parameter from a total of three types of intra prediction{2N×2N, N×N} and inter prediction {2N×2N}. For a CU whose size is lessthan minInterPredUnitSize, the encoding controller 107 selects theoptimum PU partition type as a prediction parameter from intraprediction {2N×2N, N×N}.

When the prediction mode of a CU to be entropy-encoded is interprediction and the CU size is less than or equal tominInterPredUnitSize, the encoding controller 107 in the exemplaryembodiment controls the entropy encoder 102 not to entropy-encodeinter_partitioning_idc.

The predictor 105 selects a prediction signal corresponding to theprediction parameters of each CU determined by the encoding controller107.

The prediction signal supplied from the predictor 105 is subtracted frominput video of each CU in a shape determined by the encoding controller107 to generate a prediction error image, and the prediction error imageis input to the transformer/quantizer 101.

The transformer/quantizer 101 frequency-transforms the prediction errorimage to obtain a frequency transform coefficient.

The transformer/quantizer 101 further quantizes the frequency transformcoefficient with a predetermined quantization step width Qs to obtain atransform quantization value.

The entropy encoder 102 entropy-encodes split_coding_unit_flag (see FIG.22) supplied from the encoding controller 107, the predictionparameters, and the transform quantization value supplied from thetransformer/quantizer 101. As mentioned above, when the prediction modeof a CU to be entropy-encoded is inter prediction and the CU size isless than or equal to minInterPredUnitSize, the entropy encoder 102 inthe exemplary embodiment does not entropy-encode inter_partitioning_idc.

The inverse transformer/inverse quantizer 103 inverse-quantizes thetransform quantization value with the predetermined quantization stepwidth Qs. The inverse transformer/inverse quantizer 103 further performsinverse frequency transform of the frequency transform coefficientobtained by the inverse quantization. The prediction signal is added tothe reconstructed prediction error image obtained by the inversefrequency transform, and the result is supplied to the buffer 104.

The multiplexer 106 multiplexes and outputs the information on theminimum inter-PU size (min_inter_pred_unit_hierarchy_depth) and outputdata of the entropy encoder 102. According to 4.1.2 Sequence parameterset RBSP syntax in NPL 2, the multiplexer 106 multiplexes log2_min_coding_unit_size_minus3 syntax andmin_inter_pred_unit_hierarchy_depth syntax aftermax_coding_unit_hierarchy_depth syntax in a sequence parameter set aslisted in FIG. 3 (base-2 log (logarithm) of a value obtained by dividingminInterPredUnitSize by the SCU size, i.e. 0 in the exemplaryembodiment). The log 2_min_coding_unit_size_minus3 syntax and themax_coding_unit_hierarchy_depth syntax are information for determiningan SCU size (minCodingUnitSize) and an LCU size (maxCodingUnitSize),respectively. MinCodingUnitSize and maxCodingUnitSize are respectivelycalculated as follows.

       minCodingUnitSize = 1<<       (log2_min_coding_unit_size_minus3 + 3)        maxCodingUnitSize =  1<< (log2_min_coding_unit_size_minus3  +  3  +max_coding_unit_hierarchy_depth)

The min_inter_pred_unit_hierarchy_depth syntax and minCodingUnitSizehave the following relation.

       min_inter_pred_unit_hierarchy_depth   =log₂(minInterPredUnitSize/minCodingUnitSize)

Based on the operation described above, the video encoding device in theexemplary embodiment generates a bitstream.

Referring next to a flowchart of FIG. 4, description is made of anoperation of writing the inter-PU partition type syntax that is afeature of the exemplary embodiment.

As shown in FIG. 4, the entropy encoder 102 entropy-encodessplit_coding_unit_flag in step S201. The entropy encoder 102 furtherentropy-encodes the prediction mode in step S202, i.e., the entropyencoder 102 entropy-encodes pred_mode syntax. When determining in stepS203 that the prediction mode of a CU to be encoded is inter predictionand determining in step S204 that the CU size is less than or equal tominInterPredUnitSize, the encoding controller 107 controls the entropyencoder 102 to skip entropy-encoding of inter_partitioning_idc syntax.When determining in step S203 that the prediction mode of the CU to beencoded is intra prediction, or when determining in step S204 that theCU size is greater than minInterPredUnitSize, the encoding controller107 controls the entropy encoder 102 to entropy-encode, in step S205, PUpartition type information on the CU to be encoded.

According to 4.1.10 Prediction unit syntax in NPL 2, the above-mentionedpred_mode syntax and inter_partitioning_idc syntax are signaled asrepresented in a list shown in FIG. 5. The exemplary embodiment featuresthat the inter_partitioning_idc syntax is signaled only in PU headers ofCUs greater in size than minInterPredUnitSize under the followingcondition: “if(currPredUnitSize>minInterPredUnitSize).”

When the CU size of the CU to be encoded is less than or equal to thepredetermined minimum inter-PU size, the video encoding device in theexemplary embodiment does not entropy-encode the inter-PU partition typesyntax in the PU header layer of the CU to be encoded to reduce thenumber of inter-PU partition type syntaxes to be signaled. Since thereduction in the number of inter-PU partition type syntaxes to besignaled reduces the percentage of the amount of code of a PU header inthe bitstream, the quality of video is further improved.

When the CU size of the CU to be encoded exceeds the predeterminedminimum inter-PU size, the video encoding device in the exemplaryembodiment sets, in a predetermined inter-PU partition type, theinter-PU partition type syntax in the PU header layer of the CU to beencoded, and entropy-encodes the inter-PU partition type so that nointer PU the size of which is less than the minimum inter-PU size willnot come into existence. The memory bandwidth is reduced by preventingany inter PU the size of which is less than the minimum inter-PU sizefrom coming into existence.

Exemplary Embodiment 3

A video decoding device in Exemplary Embodiment 3 decodes a bitstreamgenerated by the video encoding device in Exemplary Embodiment 2.

The video decoding device in this exemplary embodiment includes: meansfor de-multiplexing minimum inter-PU size information multiplexed into abitstream; CU size determination means for determining a predeterminedCU size, from which an inter-PU partition type is parsed, based on thede-multiplexed minimum inter-PU size information; and parsing means forparsing the inter-PU partition type from the bitstream in the CU sizedetermined by the CU size determination means.

As shown in FIG. 6, the video decoding device in the exemplaryembodiment includes a de-multiplexer 201, an entropy decoder 202, aninverse transformer/inverse quantizer 203, a predictor 204, a buffer205, and a decoding controller 206.

The de-multiplexer 201 de-multiplexes an input bitstream and extractsminimum inter-PU size information and an entropy-encoded videobitstream. The de-multiplexer 201 de-multiplexes log2_min_coding_unit_size_minus3 syntax andmin_inter_pred_unit_hierarchy_depth syntax aftermax_coding_unit_hierarchy_depth syntax in sequence parameters as listedin FIG. 3. The de-multiplexer 201 further uses the de-multiplexed syntaxvalues to determine a minimum inter-PU size (minInterPredUnitSize), inwhich the inter-PU partition type syntax (inter_partitioning_idc syntax)is transmitted, as follows.

       minInterPredUnitSize  =  1<< (log2_min_coding_unit_size_minus3  + 3  + min_inter_pred_unit_hierarchy_depth)

In other words, the de-multiplexer 201 in the exemplary embodiment alsoplays a role in determining the CU size, in which the inter-PU partitiontype syntax is parsed, based on the de-multiplexed minimum inter-PU sizeinformation.

The de-multiplexer 201 further supplies the minimum inter-PU size to thedecoding controller 206.

The entropy decoder 202 entropy-decodes the video bitstream. The entropydecoder 202 supplies an entropy-decoded transform quantization value tothe inverse transformer/inverse quantizer 203. The entropy decoder 202supplies entropy-decoded split_coding_unit_flag and predictionparameters to the decoding controller 206.

When the prediction mode of a CU to be decoded is inter prediction andthe CU size is minInterPredUnitSize, the decoding controller 206 in theexemplary embodiment controls the entropy decoder 202 to skipentropy-decoding of the inter-PU partition type syntax of the CU to bedecoded. The decoding controller 206 further sets, to 2N×2N, theinter-PU partition type of the CU to be decoded. When the CU size of theCU to be decoded is less than minInterPredUnitSize, the prediction modeof the CU is only intra prediction.

The inverse transformer/inverse quantizer 203 inverse-quantizestransform quantization values of luminance and color difference with apredetermined quantization step width. The inverse transformer/inversequantizer 203 further performs inverse frequency transform of afrequency transform coefficient obtained by the inverse quantization.

After the inverse frequency transform, the predictor 204 generates aprediction signal using an image of a reconstructed picture stored inthe buffer 205 based on the prediction parameters supplied from thedecoding controller 206.

The prediction signal supplied from the predictor 204 is added to areconstructed prediction error image obtained by the inverse frequencytransform performed by the inverse transformer/inverse quantizer 203,and the result is supplied to the buffer 205 as a reconstructed picture.

The reconstructed picture stored in the buffer 205 is then output as adecoded image.

Based on the operation described above, the video decoding device in theexemplary embodiment generates a decoded image.

Referring next to a flowchart of FIG. 7, description is made of anoperation of parsing the inter-PU partition type syntax that is afeature of the exemplary embodiment.

As shown in FIG. 7, the entropy decoder 202 entropy-decodessplit_coding_unit_flag to decide the CU size in step S301. In step S302,the entropy decoder 202 entropy-decodes the prediction mode. In otherwords, the entropy decoder 202 entropy-decodes pred_mode syntax. Whendetermining in step S303 that the prediction mode is inter predictionand determining in step S304 that the decided CU size is less than orequal to minInterPredUnitSize, the decoding controller 206 controls theentropy decoder 202 in step S305 to skip entropy-decoding of theinter-PU partition type and to set the PU partition type of the CU to2N×2N (inter_partitioning_idc=0).

When determining in step S303 that the prediction mode is intraprediction, or when determining in step S304 that the decided CU size isgreater than minInterPredUnitSize, the decoding controller 206 controlsthe entropy decoder 202 in step S306 not to skip entropy-decoding of thePU partition type of the CU to be decoded and to set the PU partitiontype of the CU to a PU partition type obtained as a result of theentropy-decoding.

The video encoding device in Exemplary Embodiment 1 and ExemplaryEmbodiment 2 can multiplex the minimum inter-PU size information(min_inter_pred_unit_hierarchy_depth) used in Exemplary Embodiment 1into a picture parameter set or a slice header as represented in a listshown in FIG. 8 or a list shown in FIG. 9. Similarly, the video decodingdevice in this exemplary embodiment can de-multiplex themin_inter_pred_unit_hierarchy_depth syntax from the picture parameterset or the slice header.

The video encoding device in Exemplary Embodiment 1 and ExemplaryEmbodiment 2 may set the min_inter_pred_unit_hierarchy_depth syntax asbase-2 log (logarithm) of a value obtained by dividing the LCU size(maxCodingUnitSize) by the minimum inter-PU size (minInterPredUnitSize),i.e., the following equation may be used.

       min_inter_pred_unit_hierarchy_depth   =log2(maxCodingUnitSize/minInterPredUnitSize)

In this case, the video decoding device in this exemplary embodiment cancalculate the minimum inter-PU size based on themin_inter_pred_unit_hierarchy_depth syntax as follows.

       minInterPredUnitSize  =  1<< (log2_min_coding_unit_size_minus3 +  3  + max_coding_unit_hierarchy_depth −min_inter_pred_unit_hierarchy_depth)

In the video decoding device in this exemplary embodiment, since nointer PU the size of which is less than the minimum inter-PU size doesnot come into existence, the memory bandwidth is reduced.

Exemplary Embodiment 4

A video decoding device in Exemplary Embodiment 4 decodes a bitstreamgenerated by the video encoding device in Exemplary Embodiment 1.

The video decoding device in this exemplary embodiment includes: meansfor de-multiplexing minimum inter-PU size information multiplexed into abitstream; and error detection means for detecting, based on thede-multiplexed minimum inter-PU size information, an error in an accessunit accessing the bitstream including a CU to be decoded. As defined in3.1 access unit of NPL 1, the access unit is the unit of storing codeddata for one picture. The error means violation of restrictions based onthe number of motion vectors allowed per predetermined area.

As shown in FIG. 10, the video decoding device in the exemplaryembodiment includes a de-multiplexer 201, an entropy decoder 202, aninverse transformer/inverse quantizer 203, a predictor 204, a buffer205, and an error detector 207.

The de-multiplexer 201 operates the same way as the de-multiplexer 201in Exemplary Embodiment 3 to de-multiplex an input bitstream and extractminimum inter-PU size information and an entropy-encoded videobitstream. The de-multiplexer 201 further determines the minimuminter-PU size and supplies the minimum inter-PU size to the errordetector 207.

The entropy decoder 202 entropy-decodes the video bitstream. The entropydecoder 202 supplies an entropy-decoded transform quantization value tothe inverse transformer/inverse quantizer 203. The entropy decoder 202then supplies entropy-decoded split_coding_unit_flag and predictionparameters to the error detector 207.

The error detector 207 performs error detection on the predictionparameters supplied from the entropy decoder 202 based on the minimuminter-PU size supplied from the de-multiplexer 201, and supplies theresult to the predictor 204. The error detection operation will bedescribed later. The error detector 207 also plays a role as thedecoding controller 206 in Exemplary Embodiment 3.

The inverse transformer/inverse quantizer 203 operates the same way asthe inverse transformer/inverse quantizer 203 in Exemplary Embodiment 3.

The predictor 204 generates a prediction signal using an image of areconstructed picture stored in the buffer 205 based on the predictionparameters supplied from the error detector 207.

The buffer 205 operates the same way as the buffer 205 in ExemplaryEmbodiment 3.

Based on the operation described above, the video decoding device in theexemplary embodiment generates a decoded image.

Referring to a flowchart of FIG. 11, description is made of the errordetection operation of the video decoding device in the exemplaryembodiment to detect an error in an access unit accessing a bitstreamincluding a CU to be decoded.

In step S401, the error detector 207 decides the CU size, the predictionmode, and the PU partition type.

In step S402, the error detector 207 determines the prediction mode of aPU of the CU to be decoded. When the prediction mode is intraprediction, the process is ended. When the prediction mode is interprediction, the procedure proceeds to step S403.

In step S403, the error detector 207 compares the PU size of the CU tobe decoded with the minimum inter-PU size. When the PU size of the CU tobe decoded is greater than or equal to the minimum inter-PU size, theprocess is ended. When the PU size of the CU to be decoded is less thanthe minimum inter-PU size, the procedure proceeds to step S404.

In step S404, the error detector 207 determines that there is an errorand notifies the outside of the error. For example, the error detector207 outputs the address of the CU to be decoded and in which the errorhas occurred.

According to the above operation, the error detector 207 detects theerror in an access unit accessing the bitstream including the CU to bedecoded.

Each of the aforementioned exemplary embodiments can be implemented inhardware or in a computer program.

An information processing system shown in FIG. 12 includes a processor1001, a program memory 1002, a storage medium 1003 for storing videodata, and a storage medium 1004 for storing a bitstream. The storagemedium 1003 and the storage medium 1004 may be different storage media,or storage areas on the same storage medium. A magnetic medium such as ahard disk can be used as the storage medium.

In the information processing system shown in FIG. 12, a program forcarrying out the function of each block (except the buffer block) shownin each of FIG. 1, FIG. 6, and FIG. 10 is stored in the program memory1002. The processor 1001 performs processing according to the programstored in the program memory 1002 to carry out the functions of thevideo encoding device or the video decoding device shown in FIG. 1, FIG.6, or FIG. 10, respectively.

FIG. 13 is a block diagram showing a main part of a video encodingdevice according to the present invention. As shown in FIG. 13, thevideo decoding device according to the present invention is a videoencoding device for encoding video using inter prediction, includingencoding control means 11 (the encoding controller 107 shown in FIG. 1as an example) for controlling an inter-PU partition type of a CU to beencoded, based on a predetermined minimum inter-PU size (PA) and a CUsize (PB) of the CU to be encoded.

FIG. 14 is a block diagram showing a main part of a video decodingdevice according to the present invention. As shown in FIG. 14, thevideo decoding device according to the present invention is a videodecoding device for decoding video using inter prediction, includingdecoding control means 21 (the decoding controller 207 shown in FIG. 6and FIG. 10 as an example) for controlling an inter-PU partition of a CUto be decoded, based on a predetermined minimum inter-PU size (PA) and asize (PB) of the CU to be decoded.

While the present invention has been described with reference to theexemplary embodiments and examples, the present invention is not limitedto the aforementioned exemplary embodiments and examples. Variouschanges understandable to those skilled in the art within the scope ofthe present invention can be made to the structures and details of thepresent invention.

This application claims priority based on Japanese Patent ApplicationNo. 2011-4964, filed on Jan. 13, 2011, the disclosures of which areincorporated herein in their entirety.

REFERENCE SIGNS LIST

-   -   11 encoding control means    -   21 decoding control means    -   101 transformer/quantizer    -   102 entropy encoder    -   103 inverse transformer/inverse quantizer    -   104 buffer    -   105 predictor    -   106 multiplexer    -   107, 108 encoding controller    -   201 de-multiplexer    -   202 entropy decoder    -   203 inverse transformer/inverse quantizer    -   204 predictor    -   205 buffer    -   206 decoding controller    -   207 error detector    -   1001 processor    -   1002 program memory    -   1003 storage medium    -   1004 storage medium

1. A video decoding device for decoding video using inter prediction,comprising: a decoding control unit which sets a partition type of aCoding Unit (CU) to be decoded to a type other than N×N which indicatesa size of a Prediction Unit (PU) obtained by dividing a CU to be decodedis a minimum size, when a prediction mode of the CU to be decoded is aninter prediction and a size of the CU to be decoded is equal to aminimum CU size.
 2. A video decoding method for decoding video usinginter prediction, comprising: setting partition type of a Coding Unit(CU) to be decoded to a type other than N×N which indicates a size of aPrediction Unit (PU) obtained by dividing a CU to be decoded is aminimum size, when a prediction mode of the CU to be decoded is an interprediction and a size of the CU to be decoded is equal to a minimum CUsize.
 3. A non-transitory computer readable medium storing a videodecoding program for decoding video using inter prediction, which whenexecuted by a processor performs a method comprising: setting apartition type of a Coding Unit (CU) to be decoded to a type other thanN×N to indicate that a size of a Prediction Unit (PU) obtained bydividing a CU to be decoded is a minimum size, in response to aprediction mode of the CU to be decoded being inter prediction and asize of the CU to be decoded being equal to a minimum CU size.